XTbc  mnlver^^  of  ^ttblcagt* 

' f I /!  ; 


DEVELOPMENT  OE  ROOT  SYSTEMS 
UNDER  DUNE  CONDITIONS 


A DISSERTATION 
SUBMITTED  TO  THE  FACULTY 
OF  THE  OGDEN  GRADUATE  SCHOOL  OF  SCIENCE 
IN  CANDIDACY  FOR  THE  DEGREE  OF 
DOCTOR  OF  PHILOSOPHY 

DEPARTMENT  OF  BOTANY 


BY 

WARREN  GOOKIN  WATERMAN 


Private  Edition,  Distributed  By 
THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 
CHICAGO,  ILLINOIS 


Reprinted  from 

The  Botanical  Gazette,  Vol.  LXVIH,  No.  i 
July,  1919 


1 


Xtbe  Ulntversitg  of  Cblcago 


DEVELOPMENT  OE  ROOT  SYSTEMS 
UNDER  DUNE  CONDITIONS 


A DISSERTATION 
SUBMITTED  TO  THE  FACULTY 
OF  THE  OGDEN  GRADUATE  SCHOOL  OF  SCIENCE 
IN  CANDIDACY  FOR  THE  DEGREE  OF 
DOCTOR  OF  PHILOSOPHY 

DEPARTMENT  OF  BOTANY 


BY 

WARREN  GOOKIN  WATERMAN 


Private  Edition,  Distributed  By 
THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 
CHICAGO,  ILLINOIS 

Reprinted  from 

The  Botanical  Gazette,  Vol.  LXVHI,  No.  i 
July,  1919 


Digitized  by  the  Internet  Archive 
in  2017  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


https://archive.org/details/developmentofrooOOwate 


ST  1.4 

Wild 


r t 


DEVELOPMENT  OF  ROOT  SYSTEMS  UNDER  DUNE 
^ CONDITIONS 

CONTRIBUTIONS  FROM  THE  HULL  BOTANICAL  LABORATORY  250 

W.  G.  Waterman 

(with  seventeen  figures)  • ‘ 

The  attention  of  the  writer  was  first  attracted  to  this  subject 
by  the  lack  of  knowledge  in  regard  to  the  causes  of  the  development 
and  consequent  extension  of  root  systems/  This  seemed  the  more 
surprising  because  this  knowledge  is  important,  not  only  from  a 
theoretical  standpoint,  but  also  because  of  its  bearing  on  the  prac- 
tical activities  of  plant  production.  All  of  the  responses  of  plants 
to  soil  conditions,  and  some  of  those  to  atmosphere,  are  closely  con- 
nected with  the  condition  of  their  root  systems,  but  in  the  past  both 
botanists  and  agriculturists  in  the  main  have  been  satisfied  to 
interpret  results  by  observation  of  the  shoots  only.  It  is  evident 
that  in  all  cases  where  the  chemical  and  physical  content  of  the 
soil  is  not  absolutely  uniform,  accurate  interpretation  of  results 
must  take  into  consideration  the  extension  and  general  condition 
of  the  roots. 

Recognizing  the  practical  difficulties  of  root  observations,  a 
locality  was  chosen  in  which  the  character  of  the  soil  and  the  scat- 
tered stand  of  the  plants  would  make  it  possible  to  observe  com- 
plete root  systems  under  natural  conditions.  It  was  soon  found 
that  the  distribution  of  nutrient  material  was  also  an  easily  observed 
and  very  significant  factor  in  this  locality.  The  region  also  proved 
to  have  great  geological  and  synecological  interest,  and  since  it  has 
been  described  rather  fully  (44)  elsewhere,  it  will  be  treated  only 
briefly  here. 

In  the  figures  the  root  systems  are  arranged  as  nearly  as  possible 
in  the  natural  position  in  a vertical  plane.  A 10  cm.  scale  was  the 

^ In  this  study  the  term  “development”  will  be  applied  to  the  process  of  increase 
in  size  and  branching  of  the  roots,  “extension”  will  refer  to  the  size  of  the  system,  and 
“distribution”  to  the  relative  position  of  the  system  and  its  parts  in  the  soil. 

Botanical  Gazette,  vol.  68]  [22 


C G / I - ^ 


IV  A TERM  A N—ROOT  S Y STEMS 


23 


1Q19] 

standard  measuring  stick.  When  missing  from  any  figure,  the 
scale  is  the  same  as  that  of  the  adjoining  figure. 

On  account  of  the  range  of  the  subject  and  the  relatively  small 
amount  of  work  done  in  it,  the  study  has  been  largely  a survey  of 
the  ground,  and  the  results  must  be  regarded  as  indications  for 
future  work  rather  than  as  final  solutions  of  the  problems. 

The  work  was  carried  on  under  the  direction  of  Dr.  H.  C. 
Cowles  and  Dr.  Wm.  Crocker,  and  grateful  acknowledgment  is 
made  to  them  for  their  advice  and  general  assistance;  as  well  as 
to  Dr.  G.  D.  Fuller,  Dr.  Sophia  H.  Eckerson,  and  Messrs.  H.  C. 
Sampson,  E.  J.  Kraus,  and  J.  T.  Buchholz. 

Synecology  of  Crystal  Lake  bar  region 

GEOGRAPHY  AND  GEOLOGY 

The  region  studied  is  located  in  the  northwest  corner  of  Benzie 
County,  Michigan,  and  comprises  a strip  of  land  about  i mile  wide 
and  5 miles  long  between  the  west  end  of  Crystal  Lake  and  Lake 
Michigan.  Geologically  it  is  probably  a harbor  bar  formed  during 
Algonquin  time  between  the  ends  of  two  glacial  ridges  extending 
from  Lake  Michigan  southeastwardly  on  either  side  of  Crystal  Lake. 
There  are  some  indications  that  this  strip  may  have^a  morainic  core, 
but  if  so  it  was  worked  over  and  its  top  leveled  off  during  the  Algon- 
quin high  water,  so  that  from  an  ecological  standpoint  the  situation 
would  be  the  same. 

Soon  after  the  recession  of  the  post-glacial  lake  waters,  the  winds 
began  the  work  of  piling  up  dunes.  Apparently  they  were  begun 
much  farther  west  on  land  since  eroded  away  by  the  lake.  The 
group  which  may  be  called  the  Point  Betsie  complex  starts  in  a 
point  on  Lake  Michigan  at  the  western  end  of  the  grounds  of  the 
Congregational  Summer  Assembly  and  spreads  like  a fan  to  the 
north,  about  2 miles  in  length  and  half  a mile  in  width  at  its  widest 
part.  At  its  southern  extremity  the  dunes  are  fixed  with  a very 
uneven  contour,  showing  dune  ridges  and  outlines  of  former  blow- 
outs, and  covered  by  a climax  forest.  Approaching  Point  Betsie 
I the  surface  is  lower  and  the  fixed  dunes  give  place  to  a complex  of 
moving  sand  containing  residual  patches  of  the  original  forest. 
Above  Point  Betsie  the  dunes  are  fixed  again  and  end  with  a definite 


24 


BOTANICAL  GAZETTE 


[JULY 


lee  slope  just  where  the  edges  of  the  northern  glacial  ridge  disappear 
under  the  level  surface  of  the  bar. 

The  glacial  ridges  consist  of  surface  deposits  of  sand  and  gravel 
more  or  less  water  washed  and  stratified,  but  contain  below  at 
least  one  layer  of  laminated  clay  several  feet  in  thickness.  ' This 
layer  appears  on  the  Michigan  shore  bluffs  and  is  occasionally 
exposed  by  erosion  on  hillsides  and  in  ravines.  On  the  shore  end  of 
the  southern  ridge  is  located  a second  small  group  of  dunes  about  half 
a mile  north  of  Frankfort.  They  are  half  a mile  in  length  and  one- 
fourth  of  a mile  in  width,  and  extend  almost  directly  north  from 
the  shore,  which  at  this  point  lies  northwest  and  southeast.  The 
group  consists  of  small  fixed  dunes  about  50  ft.  in  height,  and  the 
ridge  is  itself  about  100  ft.  above  Lake  Michigan.  These  fixed 
dunes  have  been  blown  out  through  the  center  in  a long  trough, 
which  ends  in  a large,  steep-sided,  semicircular  blowout  popularly 
called  the  Crater.”  For  100  yards  or  so  from  the  edge  of  the  shore 
bluff  the  sand  has  been  blown  away  to  or  below  the  level  of  the 
glacial  deposits,  which  are  exposed  in  the  bottom  of  the  trough. 

CHARACTER  OF  ENVIRONMENTAL  FACTORS 

Climatological. — So  far  it  has  been  possible  to  obtain  only 
incomplete  and  not  entirely  satisfactory  observations,  so  that 
only  a brief  general  statement  will  be  given.  There  is  nothing 
exceptional  about  the  meteorological  conditions  of  this  region  as  to 
temperature,  precipitation,  or  moisture  in  the  air  and  in  the  soil. 
On  account  of  the  marked  projection  of  Point  Betsie  into  Lake 
Michigan,  it  is  exposed  both  to  southwesterly  and  northwesterly 
winds,  which  probably  accounts  for  the  large  amount  of  moving 
sand  around  the  Point.  The  wind  also  has  an  indirect  influence 
on  evaporation  and  temperature,  especially  in  summer,  as  a marked 
difference  in  both  is  observed  when  a period  of  easterly  winds  is 
followed  by  a similar  period  of  westerly  winds. 

Substratum. — Open  dunes. — On  the  dunes  the  blown  sand  is 
generally  homogeneous  in  physical  character,  but  a marked  char- 
acteristic is  the  large  percentage  of  calcium  carbonate  present  in 
the  form  of  residual  grains  formed  by  the  grinding  up  of  shells, 
apparently  chiefly  of  gastropods.  This  calcium  carbonate  content 


WA  TERM  A N—ROOT  S \ ^ STEMS 


25 


1919] 

varies  from  i to  5 per  cent.  The  dune  sand  is  also  characterized  by 
a very  unequal  distribution  of  organic  material,  in  the  presence  not 
only  of  old  soil  lines,  but  also  of  buried  plant  remains  and  of  patches 
and  layers  distinctly  different  in  appearance  and  character  from 
the  ordinary  dune  sand.  The  old  soil  lines,  which  are  familiar  to  all 
who  have  any  acquaintance  with  dune  regions,  are  usually  quite 
extensive,  but  generally  appear  only  as  a dark  layer  on  the  sloping 
sides  of  blowouts.  Only  occasionally  do  they  occur  parallel  to  the 
surface  of  the  sand  at  such  a depth  as  to  affect  the  roots  of  annuals 
or  young  perennials.  The  upper  layers  of  the  sand,  however,  are 
generally  characterized  by  the  occurrence  of  layers  of  a dark  color, 
usually  a few  millimeters  in  thickness  and  covering  less  than  a square 
meter  in  area.  The  cause  of  the  color  has  not  been  determined 
definitely.  It  is  apparently  carbonaceous  in  nature,  and  its  source 
might  be  attributed  to  thoroughly  decayed  organic  matter,  and 
sometimes  also  to  unusual  deposits  of  soot  from  steamers  passing  on 
the  lake.  There  is  some  evidence  that  this  soot  accumulates 
on  the  snow  in  winter  and  is  left  on  the  sand  when  the  snow  melts 
in  the  spring.  It  has  been  noticed  also  that  in  periods  of  extreme 
dryness  a very  fine  powder  is  formed  from  the  attrition  by  the  wind 
of  the  exposed  dead  roots  and  stems  of  plants,  and  this  is  unevenly 
distributed  by  the  wind  over  large  portions  of  the  bare  sand.  When 
fixed  by  a sudden  shower  and  covered  by  later  deposits  of  blown 
sand,  very  thin  interbedded  dark  layers  might  be  produced.  Other 
layers  and  patches  are  marly  in  nature,  and  appear  to  have  been 
deposited  in  beds  of  dried  pools  in  former  pannes.  Very  rarely 
patches  of  rusty  color,  giving  an  iron  reaction,  are  found.  They 
are  only  a few  centimeters  in  length  and  suggest  a buried  nail  or 
possibly  a bit  of  meteoric  iron  as  their  cause. 

Glacial  deposits, — The  upper  layers  of  the  glacial  deposits  are 
composed  mainly  of  outwash  material,  sand  with  a few  pebbles  and 
an  occasional  bowlder.  They  are  generally  covered  with  humus, 
but  in  the  crater  blowout  the  erosion  has  been  carried  down  to  a 
gravel  layer.  This  differs  greatly  from  the  dune  sand  in  physical 
and  probably  in  chemical  constitution. 

Humus. — The  forested  dunes  as  well  as  the  glacial  deposits 
are  completely  covered  with  a thin  layer  of  leaf  mold.  This  is 


26 


BOTANICAL  GAZETTE 


[JULY 


surprisingly  uniform  in  depth,  rarely  exceeding  lo  cm.  in  thickness. 
The  sand  below  this  layer  has  been  discolored  for  several  decimeters 
and  its  chemical  content  is  plainly  affected  by  materials  leached 
down  from  the  humus  above. 

Moisture  content. — The  moisture  content  of  the  soil  varies  with 
the  location  and  the  character  of  the  substratum,  as  is  shown  in 
table  I.  The  7 cm.  samples  were  usually  taken  in  the  sand  just 
below  the  lower  edge  of  the  humus,  as  it  was  thought  that  a centi- 
meter of  depth  more  or  less  would  have  less  significance  than  an 
indeterminable  admixture  of  humus. 


TABLE  I 


Locality 

Depth  in  cms. 

Wilting 

coefficient 

Average  moisture 
content  8 weeks  July 
and  August  igi6 

Open  dune  summit 

7 

0-5 

2 0. 

a a u 

25 

0.5 

2-5  * 

Forested  dune  summit 

7 

1.8 

2 . I 

a a u 

25 

0.3 

1-3 

Forested  dune  side 

7 

1 .0 

2 . 2 

a u a 

25 

0-3 

I . 2 

Glacial  moraine 

7 

3-3 

7-5 

u a 

25 

2.4 

5-0 

ECOLOGY 

Climax  forest. — The  whole  region,  including  at  least  part  of 
the  moving  area,  was  originally  covered  by  a heavy  climax  forest, 
which  is  still  practically  untouched  in  the  southern  tip  and  along 
most  of  the  eastern  edge  of  the  dunes.  The  level  ground  on  the 
Bar  has  largely  been  cleared,  and  is  covered  with  second  growth 
of  forest  trees  and  clearing  pioneers,  where  not  occupied  by  summer 
cottages.  The  climax  forest  is  composed  of  beech,  maple,  and 
hemlock,  with  much  yellow  birch.  The  trees  are  tail  and  slender, 
with  close  stand  and  very  little  undergrowth  where  undisturbed. 
Occasional  specimens  of  Quercus  ruhra,  Finns  Strobus,  and  P. 
resinosa  are  found.  Among  the  shrubs  Acer  spicatum,  about  at 
the  southern  limit  of  its  range.  Viburnum  acerifolium,  and  Taxus 
canadensis  are  conspicuous.  Characteristic  species  in  the  under- 
growth are  Aralia  nudicaulis,  A.  racemosa,  Streptopus  roseus, 
Clintonia  borealis,  Maianthenium  canadense,  Linnaea  borealis,  and 


WA  TERM  A N—ROOT  S Y STEMS 


27 


1919] 

Mitchella  re  pens,  with  Aspidium  spinulosum,  Adiantum  pedaliini, 
and  Botrychium  virginianum. 

Burned  area. — The  northern  and  central  portions  of  the  forested 
strip  have  been  burned,  in  some  parts  repeatedly,  in  others  not  so 
recently.  In  the  much  burned  portions  the  tree  specimens  are 
young  and  somewhat  stunted.  In  the  other  portions  the  trees  are 
larger  and  the  undergrowth  thicker.  The  species  include  Betida 
'alba,  Prunus  virginiana,  and  P.  pennsylvanica,  with  the  more' 
xerophytic  relics  of  the  mesophytic  undergrowth,  and  much  Pteris 
and  Equisetum. 

Border  zone  formation. — Where  the  climax  forest,  still  untouched, 
extends  to  the  shore,  a zone  50-100  yards  in  width  shows  a very 
characteristic  difference  in  species.  The  trees  are  Thuja,  Ostrya, 
Tilia,  and  Abies  halsamea,  with  Celastrus  scandens.  The  line  of 
demarcation  is  not  sharp,  but  the  climax  trees,  especially  hemlock, 
mingle  with  the  others  almost  to  the  edge  of  the  cliffs.  The 
characteristic  border  zone  species  are  not  found  farther  back  in  the 
climax  forest. 

Dune  complex  vegetation. — On  the  open  dune  complex 
there  are  found  a number  of  forest  patches,  apparently  growing  in 
valleys  between  former  fixed  dunes  whose  summits  have  been 
entirely  blown  away.  The  interiors  of  these  patches  present  all 
the  characteristics  of  a heavy  forest,  and  their  evaporation  rate  is 
almost  as  low  as  that  of  the  clim.ax  forest,  but  the  vegetation  is 
characteristic  of  the  border  zone  already  described,  containing 
especially  Thuja  and  Abies,  and  is  marked  by  some  trees  reaching 
2 ft.  in  diameter,  but  not  over  30  ft.  in  height.  The  undergrowth 
is  similar  to  that  of  the  climax  forest,  but  is  especially  characterized 
by  Viburnum  acerifolium,  Rhus  toxicodendron,  and  Aralia  nudicaulis. 
On  the  edges,  next  to  the  open  sand,  are  found  Arctostaphylos, 
Linnaea,  and  Juniperus  horizontalis . These  apparently  originate 
in  the  fixed  area  and  extend  out  onto  the  sand,  forming  a protective 
covering,  which  frequently  contains  also  Juniperus  communis. 
Buried  trees  and  occasional  graveyards  are  to  be  found  all  over  the 
moving  sand  area. 

- There  is  not  much  forest  reproduction  on  the  moving  sand,  and, 
unlike  some  similar  regions,  there  are  no  young  stands  of  Pinus 


28 


BOTANICAL  GAZETTE 


[JULY 


Banksiana,  nor  any  cottonwood  dunes  similar  to  those  of  the 
Indiana-Michigan  region.  There  are  practically  no  panne  colonies, 
but  a few  isolated  oval  groups,  chiefly  of  Thuja  and  Betula  alba, 
which  seem  to  have  originated  from  pannes,  growing  upward  as  the 
sand  accumulates  around  their  stems.  A few  other  patches  have 
reached  the  low  conifer  stage,  but  seem  chiefly  to  have  been  inva- 
sions from  the  relic  patches  previously  described.  On  the  open 
sand  the  vegetation  consists  of  characteristic  pioneer  herbs,  Am- 
mophila  and  Calamovilfa  among  grasses,  with  Lathyrus  maritima, 
Artemisia  caudata,  Campanula  rotundifolia,  Cirsium  Pitcheri,  some 
Hudsonia,  and  Zygadenus  cMoranthus . There  are  frequent  mounds 
protected  by  Calamovilfa,  Prunus  pumila,  Salix  syrticola,  and 
Cornus  stolonifera. 

The  growth  of  grasses,  especially  Ammophila,  is  quite  extensive, 
and  frequently  approaches  the  character  of  fixed  grass  dunes.  This 
is  especially  noticeable  on  the  advancing  lee  slopes,  where  the  com- 
plex is  overwhelming  the  climax  forest. 

Literature 

In  view  of  the  state  of  our  knowledge  of  the  general  subject,  the 
literature  was  reviewed  rather  fully,  but  only  a brief  summary  of  the 
results  will  be  given  here.  The  survey  covered  only  the  extension 
and  distribution  of  soil  roots,  and  the  questions  of  absorption, 
structure,  and  effects  of  environmental  factors  were  considered  only 
as  they  affected  extension.  Owing  to  the  range  of  the  subject,  the 
matter  was  treated  from  the  standpoint  of  lines  of  work  followed 
rather  than  that  qf  historical  development.  These  will  be  sum- 
marized and  general  conclusions  indicated. 

Intensive  study  oe  root  systems.— This  line  has  been  fol- 
lowed mostly  by  German  workers,  and  was  directed  chiefly  toward 
the  questions  of  structure  and  function,  either  in  different  roots  of 
the  same  maturity  or  in  the  same  root  at  different  stages  of  its 
development.  The  leading  workers  along  this  line  were  voN 
Alten  (i),  Freidenfeldt  (i6,  17),  and  Tschirch  (43).  Kroemer 
(24)  has  made  the  most  thorough  study  of  the  ‘^biological”  sig- 
nificance of  structure,  and  concludes  (i)  that  the  root  is  divided 
longitudinally  into  zones  characterized  by  greater  or  less  suberiza- 


WA  TERM  A N—ROOT  SYSTEMS 


29 


1919] 


tion  in  the  different  layers  of  the  root,  and  (2)  that  the  distances  of 
these  zones  from  the  root  tip  are  specific.  Specialized  types  are 
also  reported:  ^^contractile”  (Rimbach  30);  ‘^deciduous”  and 
“rudimentary”  (Cannon  6).  Root  systems  are  classified  as 
“intensive”  and  “extensive”  by  Busgen  (4),  and  he  regards  these 
as  hydrophytic  and  xerophytic,  respectively. 

Extension. — The  largest  amount  of  agricultural  work  has 
been  done  by  King  (23),  Ten  Eyck  (39),  Shepherd  (36),  and  Goff 
(18):  Their  conclusions  were  largely  along  practical  lines  and  in 
many  cases  simply  confirmed  conclusions  already  reached  by  empiri- 
cal methods.  Schulze’s  (34)  work  is  especially  to  be  commended 
for  carefulness  of  records  and  character  of  photographs.  Cannon 
(6,  9)  has  done  the  most  comprehensive  work  on  and  regions.  His 
records  are  very  complete,  but  his  method  of  recording  is  not  uni- 
form and  no  scales  were  photographed  with  his  plants.  In  several 
cases  photographs  manifestly  of  different  enlargement  were  included 
in  the  same  plate.  On  other  wild  herbs  the  best  work  was  by 
Weaver  (45)  on  about  20  species  of  prairie  plants.  The  observa- 
tions were  carefully  made  and  well  recorded,  and  the  plants  were 
photographed  with  scales  attached.  This  work  promises  well  for 
his  “Ecology  of  roots”  (46),  soon  to  be  published  by  the  Carnegie 
Institution.  This  latter  study  was  undertaken  to  determine  the 
root  habits  of  dominant  and  subdominant  plants  that  were  growing 
under  a wide  range  of  climatic  and  edaphic  conditions;  to  find 
the  root  relations  of  the  plant  communities  as  units  of  vegetation; 
and  to  determine  the  root  distribution  and  root  competition  of  the 
individual  species  in  their  relation  to  other  species  in  the  vegeta- 
tional  group.  Other  aims  were  to  determine  the  relation  between 
the  root  habits  of  plants  in  various  communities  and  their  succes- 
sional  sequence;  and  to  obtain  a more  definite  knowledge  of  the 
indicator  value  and  significance  of  various  species  used  in  classifying 
lands  for  grazing  or  for  agriculture;  as  well  as  to  aid  the  forester  in 
selecting  sites  for  afforestation  or  reforestation.  The  investigation 
extended  over  4 years,  during  which  time  more  than  1150  individual 
root  systems  of  about  160  species  of  shrubs,  grasses,  and  non-grassy 
herbs  were  excavated  and  studied  on  the  prairies  of  eastern  Nebraska, 
the  chaparral  of  southeastern  Nebraska,  prairies  of  southeastern 


30  BOTANICAL  GAZETTE  [july 

Washington,  plains  and  sand  hills  of  Colorado,  the  gravel  slide, 
half  gravel  slide,  and  forest  communities  of  the  Rocky  Mountains  of 
Colorado.  Among  other  interesting  observations  on  roots  inci- 
dental to  studies  of  other  features,  Hitchcock  (21),  Pammel  (27), 
and  Sherff  (37)  might  be  mentioned.  Very  little  work  has  been 
done  on  the  root  systems  of  trees.  Busgen  (5)  was  not  accessible 
to  the  writer.  Pulling  (28)  describes  the  root  systems  of  certain 
trees  of  the  northern  coniferous  forest,  but  does  not  attempt  much 
explanation  of  the  phenomena  observed. 

Effects  of  environment. — The  main  records  of  the  influence  of 
soil  on  the  extension  of  root  systems  are  incidental  to  other  sub- 
jects, as  Cannon  (6,  8),  Hilgard  (20),  Hellriegel  (19),  and 
others.  Craig  (12)  observed  the  effects  of  frost  on  the  root  sys- 
tems of  fruit  trees;  Tolsky  (40)  made  some  careful  experiments 
with  oats  grown  at  25°  and  8°,  respectively;  and  Transeau  (42) 
observed  the  results  of  different  temperatures  on  the  growth  of 
seedlings  in  bog  water.  Cannon  (10)  discusses  the  relation  of 
temperature  to  rate  of  root  growth.  As  to  the  depth  of  water  table. 
Cannon  (7)  made  observations  on  the  root  systems  of  desert  plants 
and  also  on  development  of  seedlings  in  relation  to  the  water  supply. 
Cannon  and  Hilgard  give  some  information  on  the  effects  of 
drought,  but  the  most  extensive  article  by  Rotmistrov  (31)  was 
accessible  only  in  the  review  in  the  Experiment  Station  Record. 
Bennett  (3^  offers  evidence  that  roots  of  certain  land  plants  are 
not  aerotropic,  and  Cannon  (8)  concludes  that  in  mesquite  and 
Fouquieria  aeration  within  limits  favors  root  growth  and  shoot 
development.  Noyes,  Trost,  and  Yoder  (26)  conclude  that 
excessive  CO2  is  detrimental  to  normal  root  development,  and 
agree  with  Cannon  and  Free  (ii)  as  to  the  importance  of  soil 
aeration. 

Since  the  work  of  Nobbe  (25)  and  Stohmann  (38),  Hoveler 
(22),  Benecke  (2),  and  Tottingham  (41)  have  done  the  most 
comprehensive  work  on  chemicals.  On  the  whole,  such  work  has 
been  chiefly  on  the  roots  of  seedlings,  and  the  question  at  once 
arises  whether  the  results  would  have  been  the  same  with  mature 
root  systems.  Benecke  quotes  Probst  to  the  effect  that  ‘‘observa- 
tions on  mature  plants  gave  results  opposite  to  those  in  plants  less 


igig] 


WA  TERM  A N—ROOT  S Y STEMS 


31 


mature,  but  even  then  the  evidence  seems  somewhat  contradictory.” 
Benecke  concludes  that,  on  the  whole,  scarcity  of  chemical  nutrients 
tended  to  increase  root  length,  calling  this  effect  ‘^hunger  etiolation.” 
Hoveler  reached  similar  conclusions  by  growing  plants  in  alternat- 
ing layers  of  sand  and  humus.  Seelhorst  (35),  by  counting  the 
number  of  roots  found  in  fertilized  and  unfertilized  patches,  decided 
that  ‘dn  the  fields  investigated,  plants  strongly  fertilized  not  only 
produced  stronger  roots,  but  also  roots  penetrating  to  lower  levels.” 
Rusche  (32)  used  various  salts  on  8 groups  of  plants  and  concluded 
that  the  different  groups  responded  somewhat  differently,  but  that 
on  the  whole  sulphates  produced  the  longest  and  nitrates  the 
shortest  root  systems.  Tottingham  made  a thorough  study  of  the 
effects  of  various  salts  on  young  wheat  plants  in  water  cultures  by 
varying  the  proportions  of  the  components  of  Knop’s  solution.  He 
included  observations  on  length  and  weight  of  roots  developed, 
but  drew  no  general  conclusions.  Dachnowski  (13,  14,  15)  and 
Rigg  (29)  conclude  that  toxins  in  bog  waters  and  in  decaying  rhi- 
zomes respectively  cause  stunting  of  roots  and  therefore  xerophily 
or  death  of  plants. 

Schreiner  and  Reed  (33)  conclude  that  roots  of  healthy  grow- 
ing plants  excrete  substances  deleterious  to  root  growth,  especially 
in  plants  of  the  same  species. 

Conclusions  from  literature 

1.  There  has  been  comparatively  little  work  done  on  extension 
of  root  systems  as  such,  and  the  value  of  the  results  is  lessened  by 
the  lack  of  uniformity  in  recording,  which  makes  it  practically 
impossible  to  compare  the  results  of  different  workers.  The  use  of 
vague  descriptive  terms  in  characterizing  the  branching  of  roots 
is  also  unsatisfactory. 

2.  Much  variability  of  roots  as  a result  of  the  action  of  the 
environment  is  reported,  but  most  of  this  action  is  destructive,  as 
the  results  of  frost,  drought,  hard  soil  layers,  etc.  The  experi- 
mentation with  chemicals  shows  differences  in  length  and  weight  of 
roots,  but  does  not  offer  any  definite  evidence  as  to  the  causes 
of  root  extension  in  general  or  of  differential  extension  within  a 
single  root  system. 


32 


BOTANICAL  GAZETTE 


[JULY 


3.  The  elaborate  studies  of  the  structure  of  roots  are  marked  in 
the  main  by  great  freedom  of  inference  as  to  the  functions  of  these 
structures.  There  would  seem  to  be  need  of  further  evidence, 
experimental  or  otherwise,  to  give  a more  certain  foundation  for  the 
statements  as  to  function. 

4.  It  seems  evident  that  extension  of  root  systems  should  be 
interpreted  in  the  light  of  the  structure  and  function  of  the  roots  in 
question.  The  length  of  a root  is  of  little  importance  unless  we 
know  how  much  of  it  is  functioning  for  the  plant.  Great  length, 
also,  or  closeness  of  branching,  may  have  very  different  causes  and 
effects  in  different  species  or  under  different  conditions,  and  so  a 
very  different  meaning  to  the  plant. 


Root  systems  of  dune  plants 

GROWTH  HABITS  OF  ROOT  SYSTEMS 

Methods. — ^From  the  consideration  of  the  literature,  it  becomes 
evident  that  the  extension  of  root  systems  means  very  little  ecologi- 
cally unless  interpreted  in  the  light  of  function  and  of  probable 
causes  of  that  extension.  Studies  which  include  simply  the  extension 
of  root  systems,  without  considering  the  conditions  of  their  develop- 
ment and  in  some  way  evaluating  the  absorbing  power  of  their 
different  parts,  omit  a large  part  of  the  significant  elements  of  the 
problem.  For  these  reasons  it  seemed  better  not  to  attempt  to 
study  the  extension  of  the  root  systems  of  all  possible  plants  found 
on  the  dunes,  but  to  confine  the  attention  more  intensively  to  a 
few  species.  Within  the  limits  of  the  present  paper  it  will  be  pos- 
sible to  study  only  the  development  of  roots  in  relation  to  the 
factors  of  their  environment.  This  can  best  be  done  by  beginning 
with  the  germinating  seed  and  tracing  the  probable  course  of 
development  with  the  influence  of  the  environmental  factors  always 
in  mind.  From  this  viewpoint  length  and  weight  of  roots  would  be 
of  less  importance  than  the  determination  of  a ‘‘normal”  root  sys- 
tem and  the  interpretation  of  the  modifications  actually  found. 
The  question  of  structure  and  function,  while  equally  important, 
cannot  be  considered  in  the  present  paper,  but  must  be  left  for 
future  study. 


IV  A TERM  A N—KOOT  S Y STEMS 


33 


1919] 

General  description  of  certain  species. — Prunus  pumila. 
— This  was  the  species  first  investigated  because,  as  a perennial 
well  distributed  over  the  Betsie  complex,  it  showed  promise  of  a 
permanent  and  well  marked  root  system.  On  investigation  it 
proved  to  have  not  only  these  features  but  other  characteristics 
which  admirably  fitted  it  to  serve  as  a basis  of  comparison  with 
other  species.  As  usually  observed  on  the  dunes,  the  plant  has 
more  or  less  of  a shrub  habit,  with  many  stems  caused  by  vegetative 
reproduction  of  parts  buried  by  the  sand.  Under  these  conditions 
it  frequently  functions  as  a sand  binder  and  forms  a protected  knoll 
or  hummock.  On  the  other  hand,  where  the  sand  is  being  blown 
away,  a long  straggling  stem  will  be  produced,  more  or  less  prostrate 
and  with  little  branching.  In  general  the  shrub  type  seems  to 
fruit  best,  and  seedlings  are  most  abundant  at  the  base  of  a shrub  or 
around  a knoll,  although  the  fruits  sometimes  roll  or  blow  some 
distance  before  being  covered  with  sand. 

The  only  conditions  of  germination  seem  to  be  burial  over  a 
winter  in  i or  2 inches  of  sand.  In  the  laboratory  the  seeds  germi- 
nated readily  in  a sterile  moist  chamber  on  filter  paper,  after  the 
stony  pit  and  the  seed  coat  had  been  removed. 

After  3 months’  growth  on  the  dunes  the  root  system  shows  the 
general  type  illustrated  in  fig.  i,  while  a 3-year-old  seedling  is 
shown  in  fig.  2.  The  root  system,  however,  does  not  usually  develop 
along  these  lines.  The  more  usual  form  is  extremely  asymmetric 
and  develops  very  unevenly,  as  indicated  in  fig.  3.  All  the  seedlings 
show  wide  individual  diversity  of  form,  and  these  specimens  are 
not  at  all  exceptional  in  their  eccentricity.  An  examination  of  these 
asymmetric  forms  shows  that  their  irregularities  are  connected  with 
the  distribution  of  more  or  less  decayed  plant  parts  under  the  sand. 
A typical  illustration  of  this  is  shown  in  fig.  4,  where  horizontal 
laterals  are  shown  passing  through  and  exploiting  bits  of  stems, 
branches,  etc.  When  a seedling  grows  in  the  vicinity  of  a dark 
layer  already  described,  its  laterals  are  generally  found  in  these 
layers,  and  they  are  more  branched  and  longer  than  those  in  ordi- 
nary dune  sand;  in  fact,  a layer  of  this  sort  will  generally  be  found 
to  contain  many  rootlets  of  various  species  of  plants,  while  the  sand 
on  either  side  is  almost  completely  free  from  them.  An  interesting  . 


/ 


34 


) 


BOTANICAL  GAZETTE 


[JULY 


case  was  found  in  the  sand  at  Miller,  Indiana,  shown  in  fig.  5. 
A 3-year-old  specimen  {h)  grew  within  the  field  of  such  a layer,  and 
the  effect  is  seen  in  its  well  developed  horizontal  laterals.  The 
carbonaceous  layer  in  question  was  absent  in  the  neighborhood  of 


Fig.  I 


Fig.  2 


Figs,  i,  2. — Pninus  pumila:  fig.  i,  3 months  old;  fig.  2,  3 years  old 


Fig.  3. — Prunus  pumila,  showing  asymmetric  habit 


IV  A TEiaiAN—ROOT  S VS  TENS 


1919] 


% 


35 


the  2-year-old  specimen  a,  barely  2 ft.  away,  and  the  absence  of 
prominent  laterals  is  very  marked.  Cases  of  extreme  elongation 
were  occasionally  observed,  as  in  fig.  6.  In  the  specimen  figured, 
the  root  came  in  contact  with  a decaying  grass  rhizome  and  turned 
back  at  a sharp  angle,  following  the  rhizome  for  about  60  cm.  In  a 
similar  case  the  root  did  not  make  such  a sharp  angle,  but  followed 
the  rhizome  for  an  even  greater  distance.  It  should  be  noted  in 


Fig.  4 


Fig.  s 


Figs.  4,  5. — Prunus  pumila:  fig.  4,  showing  relation  of  roots  to  buried  plant  parts; 
fig.  $a,  2 years  old,  which  grew  in  normal  dune  sand;  b,  same,  -3  years  old,  which  grew 
in  sand  with  interbedded  black  layers. 


both  cases  that  the  size  of  the  shoot  was  not  at  all  in  proportion  to 
the  length  of  the  root. 

In  these  cases  the  relation  of  the  root  to  the  organic  matter  is  not 
clear.  Generally  there  is  little  penetration  of  the  tissue  of  the 
decaying  organ.  There  are  occasionally  short  laterals  clasping 
the  foreign  body,  frequently  passing  under  a sheathing  leaf  or  a 
disorganized  epidermis,  and  in  cases  of  extreme  decay  adjusting 
themselves  to  the  easy  passages  formed  by  the  disintegration  of  the 
middle  lamella  of  the  cell  walls.  There  are  no  indications  of  haus- 
toria  or  of  actual  penetration  of  cell  walls.  In  black  layers  and 


36 


BOTANICAL  GAZETTE 


[JULY 


patches  in  the  sand  there  are  cases  of  extreme  development  of  small 
rootlets  with  close  branching. 

The  development  of  the  shoot  is  more  or  less  connected  with  the 
presence  of  these  plant  parts.  Seedlings  whose  roots  do  not  come 
in  contact  with  such  organic  matter  have  few  and  small  leaves,  and 
in  general  it  would  seem  that  the  securing  of  food  from  such  organic 
matter  is  essential  to  the  development  and  maturity  of  the  plant. 
Most  of  the  plants  which  have  reached  the  shrub  stage  are  found  to 

have  stems  which 
have  apparently 
come  up  through 
superposed  masses 
of  sand,  and  while 
it  has  not  been 
possible  to  demon- 
strate the  presence 
of  organic  matter 
at  the  base  of  such 
shrub  systems, 
such  presence  is 
very  strongly  con- 
noted by  the  ap- 
pearance of  the 
plant  and  the  evidence  from  the  smaller  specimens  excavated.  The 
condition  of  the  shoots  with  very  long  roots  will  be  considered  later. 

In  the  sphere  of  inhibiting  factors  very  little  evidence  was 
observed.  On  account  of  the  scattered  stand  the  roots  seldom 
come  into  contact  with  roots  of  other  plants,  but  occasionally 
they  were  found  exploiting  bits  of  buried  plant  material  in  com- 
pany with  and  unaffected  by  roots  of  (a)  other  species.  In  one 
case,  however  (fig.  7),  a P.  pumila  seedling  had  sent  its  roots  down 
almost  into  a thick  mat  of  willow  roots  {h).  Here  there  seems  to 
have  been  a very  definite  dwarfing  of  the  root  system,  either  from 
the  presence  of  injurious  excretions  or  because  of  the  removal  of 
water  or  nutrient  material  by  the  willow  roots. 

The  presence  or  absence  of  the  water  table  seemed  to  have  no 
directive  effect  on  the  P.  pumila  roots;  in  fact  several  seedlings 


Fig.  6. — Primus  pumila  with  long  lateral  developed 
in  contact  with  dead  Ammophila  rhizome. 


igig]  WATERMAN— ROOT  SYSTEMS  37 

were  found  growing  in  the  water  on  the  edge  of  a pool,  with  no 
apparent  effect  on  the  roots.  Observations  near  Miller,  Indiana,  on 
P.  pumila  and  Populus  deltoides,  seemed  at  first  to  show  such  effects 
(fig.  8) . The  seedlings  were  growing  in  very  wet  sand  with  the  water 
table  only  8-10  cm.  below  the  surface.  On  inspection  the  water  table 
zone  proved  to  be  very  mucky  and  foul,  and  the  effects  on  the  tap- 
roots seemed  to  be  a rotting  due  to  the  action  of  micro-organisms, 
with  a proportionately  increased  development  of  laterals. 


Fig.  7 Fig.  8 


Figs.  7, 8. — Pnmus  pumila:  fig.  la,  with  stunted  root  system;  h,  horizontal  layer 
of  matted  willow  roots-in  approximately  natural  position  with  relation  to  P.  pumila 
roots;  fig.  8,  root  systems  showing  effects  of  high  water  table,  Miller,  Indiana:  a, 
Populus  deltoides;  b and  c.  Primus  pumila. 

Ammophila  arenaria. — While  this  is  the  typical  plant  of  the 
open  dunes,  its  seedlings  are  difficult  to  find,  and  its  reproduction 
is  mostly  vegetative  through  rhizomes.  The  spikes  are  thoroughly 
exploited  by  insects,  and  when  gathered  late  in  the  season  very 
few  seeds  will  be  found  untouched.  For  this  reason  the  seedlings 
are  very  scanty,  except  in  dense  colonies  where  mature  spikes 
have  been  buried  by  fresh  sand,  and  even  then  there  are  seldom  over 
6 seedlings  from  a whole  spike.  In  the  mature  plant,  as  is  well 
known,  there  is  a long  slender  rhizome  producing  usually  2 roots 


38 


BOTANICAL  GAZETTE 


[JULY 


and  a-  bud  at  each  node.  The  bud  frequently  does  not  develop, 
but  the  roots  range  widely  under  normal  conditions  through  fresh 
blown  sand,  often  reaching  a length  of  several  meters.  They  are 
thickly  set  with  short  laterals  which  bear  abundant  root  hairs. 
At  times  the  tips  of  these  roots  are  much  enlarged  for  about  lo  cm., 
gradually  tapering  in  both  directions  and  without  branches.  All 
the  roots  are  extremely  wiry  and  tenacious,  and  on  account  of  their 
fineness  and  length  cannot  well  be  shown  in  a figure. 

It  has  generally  been  recognized  (Westgate  47)  that  Ammoph- 
ila  ordinarily  requires  annual  supplies  of  fresh  sand  for  its  best 

development.  Field  ob- 
servations in  this  region 
would  indicate  also  that  it 
does  not  thrive  in  sand 
possessing  an  appreciable 
amount  of  humus,  and  that 
its  colonies  do  not  extend 
across  the  border  into  such 
a region  (sand  containing 
humus).  In  at  least  one 
case  a colony  seems  to  be 
dying  out  in  the  presence 
of  a competing  colony  of 
Calamovilfa,  which  has 
not  been  studied  closely, 
but  which  seems  to  be 
more  humus  tolerant  than 
Ammophila.  Excavation 
shows  that  the  roots  of  mature  plants  of  Ammophila  exhibit  little 
response  to  organic  material,  following  black  layers  only  slightly  and 
frequently  passing  obliquely  through  them.  A study  of  the  seedlings 
would  seem  to  indicate  that  the  extension  of  their  roots  may  to  some 
extent  be  inhibited  by  the  presence  of  decaying  plant  parts.  In 
fig.  9,  a grew  in  pure  sand,  but  in  h all  the  roots  came  directly  into 
contact  with  buried  Ammophila  leaves  and  stems,  and  in  c the 
upper  roots  developed  in  pure  sand  and  the  lower  in  plant  remains. 
It  will  be  noted  that  in  h and  c the’ roots  do  not  extend  beyond  the 


Fig.  9. — -Seedlings  of  Ammophila  arenaria, 
2-3  months  old:  a,  in  homogeneous  dune  sand; 
b,  all  roots  in  contact  with  buried  plant  parts;  c, 
upper  roots  in  sand,  lower  in  contact  with  buried 
plant  parts. 


1919] 


WA  TERM  A N—ROO  T SYS  TEMS 


39 


decaying  plant  parts.  Great  care  was  taken  in  excavating-  these 
specimens,  which  are  simply  typical  of  a number  of  cases  found,  and 
there  can  be  no  question  that  in  every  case  the  roots  stopped  in  the 
decaying  plant  parts  as  figured. 

Artemisia  caudata. — This  species  germinates  freely  on  the  open 
dunes,  and  also  in  the  edge  of  the  forested  sections.  It  is  described 
by  Gray  as  not  perennial,  but  the  character  of  the  mature  speci- 
mens found  on  the  dunes  would  indicate  that  there  at  least  it  has  a 
perennial  habit.  Two  series  of  plants  are  shown,  one  grown  in 
pure  sand  (fig.  10),  the  other  in  sand  containing  some  admixture 


Figs.  10,  ii. — Artemisia  caudata:  fig.  10,  which  grew  in  pure  dune  sand,  showing 
stumps  only  of  long  laterals;  fig.  ii,  of  apparently  same  ages  as  in  fig.  lo,  but  in  sand 
containing  some  humus;  laterals  very  short. 

of  humus  (fig.  ii).  In  the  former  may  be  observed  the  stubs  of  the 
characteristic  laterals,  which  in  mature  specimens  extend  20-30  ft. 
No  attempt  has  been  made  to  show  in  the  figure  the  full  extent 
of  these  laterals,  but  rather  their  relation  to  each  other  and  to  the 
taproot.  In  pure  sand  this  species  shows  some  of  the  asymmetry 
of  Prunus  pumila,  but  the  causes  are  not  so  evident.  In  some 
specimens  very  long  laterals  develop  almost  on  the  surface  of  the 
sand,  and  are  so.  shallow  that  they  are  oftei;  exposed  and  killed 
by  the  blowing  away  of  the  upper  dry  sand  layer.  In  other  speci- 
mens it  is  one  or  more  of  the  deep  seated  laterals  which  shows 


40 


BOTANICAL  GAZETTE 


[JULY 


extreme  elongation.  The  superficial  laterals  show  marked  paral- 
lelism with  the  upper  surface  of  the  sand,  even  in  plants  growing 
on  slopes;  where  the  laterals  join  the  main  root  at  different  angles, 
they  are  acute  on  the  upper  side  and  correspondingly  obtuse  on 
the  lower.  The  position  of  these  laterals  seems  to  be  on  the  rela- 
tively constant  plane  separating  the  upper  layer  of  dry  sand  from 
the  moist  sand  which  constitutes  the  remainder  of  the  substratum. 
In  general  the  system  is  characterized  by  extreme  shortness  of 
taproot,  but  occasionally  the  taproot  becomes  very  prominent. 


Figs.  12,  13. — Fig.  12,  Campanula  rotundifolia:  a,  normal  root  habit;  b,  young 
plants  on  exposed  edge  of  horizontal  marly  layer;  fig.  13,  seedlings  of  Lathyrus  mari- 
tima,  showing  remains  of  seed  and  root  tubercles. 


Artemisia  resembles  Ammophila  in  the  reduction  in  length  of  roots 
in  the  presence  of  organic  matter  or  humous  layers,  and  its  roots  are 
not  attracted  by  decaying  plant  parts.  It  will  be  noted  in  fig.  1 1 
that  the  laterals  of  this  series,  which  grew  in  sand  mixed  with  humus, 
are  finer,  more  branched,  and  with  none  of  the  very  long  laterals 
of  the  pure  sand  specimens.  The  fact  that  the  roots  are  not 
attracted  by  decaying  plant  parts  was  distinctly  shown  in  one  case 
where  a mature  plarwt  with  laterals  up  to  6 ft.  in, length  was  growing 
near  a decaying  log.  One  lateral  passed  a few  inches  below  the 
log  in  a zone  of  leaching,  bent  first  away,  then  toward  the  log. 


IV  A TERM  A N—ROOr  S Y STEMS 


41 


1919] 

then  away  again,  and  showed  in  general  no  effect  attributable  to 
the  presence  of  the  log.  A marked  attraction  would  have  been 
expected  if  the  plant  had  been  P.  pumila. 

Cirsium  Pitcheri. — This  is  apparently  ecologically  similar  to 
Artemisia  caudata  as  to  locality  and  general  conditions  of  growth, 
but  it  differs  in  having  a strongly  developed  taproot  with  very  few 
and  inconspicuous  laterals.  Occasionally,  for  no  apparent  reason, 
a single  lateral  becomes  prominent,  as  in  Artemisia. 

Campanula  rotundifolia. — This  species  is  not  uniformly  dis- 
tributed, but  apparently  germinates  freely  in  restricted  localities 
on  the  open  dunes,  as  well  as  in  open  places  of  a humus  bearing 
substratum.  Its  marked  characteristic  on  the  dunes  is  the  long 
taproot  with  U-shaped  insertion  of  laterals  (fig.  12a).  In  one  case 
where  a buried  marly  layer  was  exposed  on  a slope  a row 
of  seedlings  was  observed  growing  along  the  exposed  edge  of  the 
layer,  and  the  roots  were  found  to  have  grown  horizontally  inward 
through  the  marly  layer  (fig.  12b).  They  have  not  been  observed 
in  contact  with  the  dark  layers,  so  their  reaction  to  them  cannot  be 
stated. 

Lithospermum  Gmelini. — This  is  a fairly  well  distributed  plant 
on  the  open  dunes,  and  is  characterized  by  a very  long,  thick,  black 
taproot,  with  almost  no  laterals.  A study  of  the  structure  might 
reveal  some  interesting  features  as  to  the  absorbing  power  of  this 
root. 

Lathyrus  maritima. — This  species  is  frequently  the  companion 
of  Ammophila,  but  it  differs  in  root  development  in  some  important 
respects.  It  germinates  freely  on  the  open  dune,  producing  numer- 
ous seedlings.  No  marked  seedlings  have  been  followed  over 
winter,  but  from  the  scarcity  of  2-year-old  plants  they  do  not  seem 
to  survive  well,  either  on  account  of  sand  movement  or  possibly 
because  of  lack  of  nutrition.  The  mature  plant  develops  a wide 
ranging  rhizome,  deeply  placed  and  difficult  to  excavate  unless 
exposed  in  a blowout.  The  roots  are  scattered  and  usually  not 
over  10-20  cm.  in  length.  Root  tubercles  are  early  developed 
in  dune  sand  (fig.  13),  but  less  in  some  locahties  than  in  others, 
possibly  on  account  of  unequal  distribution  of  infecting  organisms. 
When  present  the  tubercles  occur  only  near  the  surface  of  the  sand. 


42 


BOTANICAL  GAZETTE 


[JULY 


they  do  not  occur  in  a humous  layer,  and  in  humus  the  roots  are 
longer  than  in  sand. 

Several  species  found  more  or  less  frequently  on  the  open 
sand,  but  especially  along  the  borders  of  forested  patches  being 
blown  out,  have  in  general  similar  characteristics,  and  will  be 
described  together.  These  are  Thuja  occidentalism  Cornus  stolonif- 
era,  Arctosiaphylos  Uva-ursi,  Vitis  spp.,  Betula  alba,  Tilia  ameri- 
cana,  and  Juniperus  horizontalis . These  rarely  germinate  on  the 
open  sand,  although  Thuja  and  Betula  alba  are  occasionally  found 
germinating  on  the  edges  of  blown  out  patches,  and  regularly 
germinate,  when  present,  on  the  floor  of  these  same  patches.  A 
study  of  their  root  systems  shows  that  the  roots  regularly  follow  and 
exploit  the  carbonaceous  layers  and  old  soil  lines,  and  the  plants 
remain  stunted  unless  their  roots  find  such  plant  remains. 

Populus  deltoides. — This  species  is  very  rare  in  this  region,  and 
its  ecological  equivalent,  P.  balsamif era,  is  found  sparingly  along 
the  shore  of  Lake  Michigan,  more  frequently  on  Crystal  Lake 
beach,  and  in  the  burned  part  of  the  forested  dune  area. 

Salix  spp. — The  willows  were  not  included  in  this  study, 
although  their  roots  were  frequently  met,  and  certain  points  may 
be  noted.  They  showed  a positive  and  vigorous  hydrotropism, 
which  was  observable  in  the  neighborhood  of  pannes  and  also  of 
Lake  Michigan.  Several  times  roots  were  found  descending  with 
the  slope  of  the  surface  of  the  sand  until  they  finally  entered  the 
water  table  and  passed  under  the  pool  in  the  panne.  They  also 
show  in  marked  degree  the  ability  to  form  a small  bunch  of  closely 
branched  rootlets  in  small  dark  patches  in  the  sand. 

GENERAL  OBSERVATIONS  ^ 

Root  systems  in  glacial  gravel  layers. — In  the  blown  out  portions 
of  the  Crater  group  gravelly  layers  of  the  glacial  substratum  had 
been  laid  bare  and  bore  a scattered  vegetation  similar  to  that  of 
the  dunes.  There  was  some  excavation  of  Prunus  pumila  and  of 
Artemisia  in  this  locality,  and  indications  seemed  to  point  to  a 
closer  and  more  regular  branching.  There  were  of  course  no 
buried  plant  parts,  so  that  asymmetric  development  would  not  be 
expected  (fig.  14). 


IVA TERMAN—R007^  SVSTEAIS 


43 


1919] 

Mycorrhiza. — The  question  of  symbiotic  fungi  suggested  itself 
early  in  the  investigations,  but  there  was  no  opportunity  for  a 
detailed  study.  Slight  examination  of  a few  species  was  made, 
but  did  not  show  evidence  of  the  presence  of  such  fungi. 

Enlarged  root  tips. — A frequently  observed  feature  of  many  of 
the  dune  species  was  the  marked  enlargement  of  some  root  tips. 
The  tip  rapidly  increased  to  3 or  4 times  its  normal  diameter  and  then 
gradually  tapered  away  again,  the  extreme  length  of  the  enlarge- 
ment being  20-30  times  its  greatest  diameter.  This  enlargement 
seemed  to  be  confined  to  the  central  tip  of  the  larger  branches  only. 


and  especially  in  those  which  showed  a marked  and  rapid  extension 
into  new  territory,  for  which  reason  the  name  ‘^pioneer”  or  invad- 
ing root  tip  has  suggested  itself  for  this  type.  It  was  found  under 
apparently  normal  conditions  both  in  pure  sand  and  in  humus,  but 
in  the  sand  it  seemed  to  be  increased  in  some  species  by  the  chemical 
solutions  used,  and  even  by  distilled  water.  The  largest  tips  found 
were  in  the  case  of  J uncus  halticus,  where  in  shape  and  size  they 
resembled  large  angleworms.  It  was  not  possible  to  make  a 
microscopic  examination  of  fresh  material,  but  sections  of  tips 
preserved  in  4 per  cent  formaldehyde  showed  no  signs  of  symbiotic 
fungi  or  animal  forms.  The  enlargement  seemed  to  be  due  to 
increase  in  size  and  number  of  the  parenchyma  cells. 


44 


BOTANICAL  GAZETTE 


[JULY 


In  several  cases  there  were  indications  that  taproots  and 
laterals  gave  different  reactions  to  the  same  stimuli.  In  these 
species  taproots  were  never  deflected  by  horizontal  black  layers  or 
plant  parts,  while  laterals  were  apparently  free  to  move  in  any 
direction.  Typical  species  showing  this  habit  were  Artemisia  and 
Cirsium  Pitcheri,  while  in  Prunus  pumila  and  notably  in  Campanula 
the  taproots,  if  such  they  could  be  called,  showed  marked  variabil- 
ity in  a horizontal  direction.  Whether  this  difference  was  structural 


Figs.  15,  16. — Fig.  15,  P.  pumila  root  system  after  2 months’  irrigation  with 
Knop’s  solution;  round  black  spot  represents  location  of  center  of  diffusion  and 
dotted  circle  apparent  limits  of  influence;  practically  all  new  roots  found  within  the 
circle;  fig.  16,  Thuja  occidentalis:  new  root  tips  grown  under  irrigation  with  Knop’s 
solution. 


or  due  to  a difference  in  quantity  or  quality  of  the  stimulus  was  not 
evident. 

EXPERIMENTATION 

A consideration  of  these  observations  would  suggest  several 
possible  factors  as  causes  for  the  observed  characteristics  of  root 
systems  of  dune  plants.  Chief  among  these  would  be  the  distri- 
bution of  chemical  materials  not  normally  found  in  dune  sand,  also 
the  distribution  in  the  sand  of  moisture  and  of  oxygen  and  the 
varying  penetrability  of  the  sand.  As  the  chemical  factor  seemed 


igig] 


WA  TERM  A N—ROO  T S YS  TEMS 


45 


most  likely  to  be  the  controlling  one,  some  experiments  were  carried 
on  in  the  endeavor  to  clear  up  this  point  if  possible,  and  to  indicate 
the  nature  of  the  chemicals  producing  the  different  results. 

Experiments  with  plants  in  situ. — The  first  question  to  be 
settled  was,  can  root  extension  be  stimulated  by  the  presentation  of 
nutrient  material  either  to  the  root  tips  or  along  the  more  mature 
roots  ? This  was  investigated  by  diffusing  Knop’s  solution  from 
porous  cylindrical  cups  (old  atmometer  cups  in  this  case)  which  had 
been  buried  in  the  sand  near  the  roots  of  growing  plants.  The  cups 


Fig.  17. — P.  pumila,  3 months’  seedlings  grown  in  pots:  a,  irrigated  with  rain 
water;  b,  with  Knop’s  solution;  c,  grown  in  rotted  barnyard  manure  and  humus. 

were  located  at  a distance  of  5-10  cm.  from  large  lateral  roots  near 
a portion  of  the  root  practically  without  secondary  laterals.  The 
cups  were  filled  about  3 times  a week  with  o . 6 Knop’s  solution  and 
were  dug  up  after  2 months.  In  every  case  there  was  a marked 
development  of  laterals  in  the  zone  of  diffusion  of  the  solution. 
This  was  tried  with  Prunus  pumila  and  with  'Thuja  occidentalis 
(figs.  15,  16).  The  contrast  between  the  old  bunched  roots  and 
the  new  slender  ones  is  striking.  Controls  with  distilled  water 
showed  no  abnormal  development. 


46 


BOTANICAL  GAZETTE 


[JULY 


Pot  cultures  with  nutrient  materials. — Pot  cultures  with 
P.  pumila  seedlings  were  grown  in  dune  sand  watered  (i)  with  dis- 
tilled water,  and  (2)  with  Knop’s  solution,  and  (3)  in  a substratum  of 
rotted  barnyard  manure.  The  results  are  shown  in  fig.  17.  While 
the  root  systems  in  (i)  and  (2)  seem  rather  similar  in  extension,  an 
examination  showed  that  (2)  was  more  closely  branched  than  (i) 
and  had  nearly  twice  as  many  absorbing  root  tips.  The  striking 
feature  of  (3)  was  the  much  greater  development  of  the  shoot. 
Attempting  to  reproduce  more  closely  the  natural  conditions,  a 
seedling  was  grown  in  thedaboratory  in  a pot  of  sterile  dune  sand  in 
which  a small  patch  of  rotted  manure  had  been  included.  Here 
the  most  marked  extension  of  rootlets  appeared  under  the  patch  of 
organic  matter  in  a space  stained  by  leaching  from  the  material 
above. 

Pot  cultures  with  inorganic  salts. — Experiments  on  the 
directive  influence  of  inorganic  salts  have  showed  so  far  only  indica- 
tions and  suggestions.  Rusche  (32)  has  already  shown  differences 
in  extension  produced  by  different  salts,  and  attempts  were  made 
to  extend  his  results  along  the  line  of  directive  influence.  The 
technical  difficulties  of  finding  suitable  seedlings  and  of  presenting 
the  chemicals  in  solution  from  one  side  only,  along  with  the  uncer- 
tainty as  to  the  direction  and  extent  of  diffusion  of  the  solution 
through  the  sand,  have  so  far  prevented  reaching  any  concludve 
results.  Indications  of  a tendency  to  react  in  accordance  with  the 
sequence  of  the  liatropic  series  were  observed  and  definite  specific 
differences  were  also  evident. 

Discussion  and  conclusions 

The  most  marked  feature  of  these  observations  is  the  varied 
reactions  of  different  species  to  apparently  identical  environmental 
conditions.  These  showed  sufficient  constancy  within  a species, 
but  with  such  differences  in  different  species  as  to  suggest  that  the 
reactions  are  specific  and  that  the  controlling  factor  is  heredity. 
Their  general  character  also  strongly  suggests  their  relation  to  a 
former  habitat  or  condition  of  growth.  It  would  seem  possible 
that  species  which  had  varied  in  the  direction  of  an  extended  root 
system  in  sandy  soil  had  survived  there  and  had  become  adjusted  to 


iQig] 


WA  TERM  A N—ROOr  SYSTEMS 


47 


that  environment,  and  that  when  germinating  in  soils  richer  in 
nutrients  these  root  systems  would  be  less  extended.  On  the  other 
hand,  species  which  had  similarly  become  adjusted  to  the  richer 
soils  had  thereby  become  dependent  on  these  soils  for  the 
development  of  extended  root  systems,  and  were  therefore  stunted 
in  sandy  soils.  While  a relation  to.  past  conditions  may  not  be  the 
true  explanation  for  this  specificity,  the  facts  are  evident  and  cer- 
tainly indicate  very  definite  relations  to  present  conditions.  These 
should  be  of  great  value  in  selecting  species  for  revegetating  exposed 
areas  and  other  localities  where  the  humous  content  of  the  soil  is 
slight  or  unevenly  distributed,  as  well  as  in  the  cultural  treatment  of 
species  which  may  be  regarded  as  worthy  of  development  for  their 
economic  value. 

On  this  basis  it  would  follow  that  general  statements  in  regard 
to  the  root  habits  of  dune  plants  as  a class  are  dangerous,  and  that 
the  so-called  ‘^dune  pioneers”  are  not  all  on  the  same  footing; 
that  in  fact  they  should  be  put  into  two  widely  different  groups  with 
a series  of  types  occupying  intermediate  positions.  In  one  group 
would  be  the  Prunus  pumila  type,  which  does  not  have  the  power 
of  extending  root  systems  widely  in  pure  dune  sand,  but  is  stunted 
and  does  not  reach  maturity  unless  its  roots  find  buried  organic 
matter.  In  the  opposite  group  would  be  the  Ammophila  type, 
which  reaches  maturity  in  pure  dune  sand  and  whose  root  system  is 
limited  in  extension  by  the  presence  of  decaying  plant  materials. 
Similar  to  Ammophila  would  be  Artemisia,  Cirsium,  and  Campanula, 
with  Calamovilfa  and  other  grasses,  and  probably  species  of  Solidago 
and  Aster.  Lithospermum  is  similar  ecologically,  although  with  a 
very  different  root  habit,  while  Lathyrus  maritima  occupies  a 
peculiar  position  on  account  of  its  relationship  to  the  nodular 
bacteria.  From  these  observations  it  would  seem  that  Ammophila 
is  the  only  plant  which  can  reasonably  be  expected  to  thrive  suc- 
cessfully on  the  normal  dune  substratum. 

In  seeking  for  the  causes  of  the  asymmetric  development  of  root 
systems  observed,  it  is  evident  that  the  only  factors  to  be  considered 
are  those  working  in  the  soil  and  exerting  an  unequal  or  one-sided 
influence  in  the  system  as  a whole.  These  may  be  limited  to  four, 
namely,  moisture,  chemicals,  oxygen,  and  density  or  penetrability 


48 


BOTANICAL  GAZETTE 


[JULY 


of  the  soil.  In  the  case  of  a few  plant  groups,  as  the  willows,  which 
are  recognized  as  having  a hereditary  hydrotropic  tendency,  there 
will  be  little  question  as  to  the  dominant  factor,  but  the  evidence 
in  the  cases  of  the  species  previously  described  must  be  considered 
somewhat  carefully. 

Taking  up  first  the  possibility  of  moisture  as  the  main  factor, 
it  should  be  noted  that  below  the  first  few  centimeters  water  is 
evenly  distributed  in  dune  sand, .and  cannot  be  regarded  as  the 
causal  factor  in  the  development  of  the  asymmetric  root  systems 
described.  It  might  be  assumed  that  the  following  of  humous  layers 
and  dark  patches  by  plant  roots  should  be  attributed  to  the  presence 
of  a greater  amount  of  moisture  in  those  patches  than  in  the  adjoin- 
ing sand.  Granting  the  power  of  humus  to  absorb  greater  amounts 
of  water  than  can  the  pure  sand,  it  would  be  difficult  to  prove  that 
water  was  held  by  these  patches  beyond  the  quantity  required  by 
their  increased  wilting  coefficients.  The  patches  are  so  small  and 
the  medium  so  unstable  that  it  would  be  practically  impossible  to 
collect  from  adjoining  patches  pure  samples  large  enough  for 
accurate  determination  of  their  respective  moisture  contents  and 
wilting  coefficients.  In  the  experimental  work  the  adjustment  of 
the  water  supply  was  a difficult  matter.  As  a result  there  were 
occasional  reactions  which  seemed  to  point  to  the  influence  of 
moisture,  but  even  in  these  it  could  not  positively  be  stated  that 
chemical  influence  was  not  the  dominant  factor  with  moisture  as 
contributory  only. 

Approaching  the  question  from  the  chemical  side,  we  find 
definite  evidence,  both  from  observation  and  from  experimentation, 
that,  with  the  species  considered,  variations  in  chemical  solutions 
produced  changes  in  root  development,  while  variations  in  water 
supply  produced  little  or  no  evidence  of  such  changes  in  develop- 
ment. Perhaps  the  most  conclusive  evidence  was  found  in  the  case 
of  a patch  of  humus  in  a pot  culture  of  Prunus  pumila.  Here 
the  abnormal  development  of  laterals  occurred  under  the  patch  of 
humus  in  a zone  stained  by  leaching  of  organic  matter  rather  than 
in  the  free  region  on  either  side,  where  the  moisture  content  should 
be  the  same  as  in  the  region  under  the  organic  patch. 

We  find  also  marked  differences  in  elongation  of  root  systems  in 
the  presence  of  decaying  plant  parts.  Contact  with  or  even  coming 


igig] 


\VA  TERM  AN— ROOT  SYSTEMS 


49 


into  the  zone  of  influence  of  these  plant  parts  seems  to  cause  elonga- 
tion of  the  roots  of  some  species,  but  inhibition  in  those  of  others. 
The  determining  factor  here  might  be  a direct  chemical  stimulation, 
the  furnishing  of  organic  material,  or  in  opposite  cases  the  presenta- 
tion of  some  injurious  or  inhibiting  chemical  product  of  decay. 
No  deciding  evidence  has  as  yet  been  secured  along  this  line.  There 
were  some  indications  that  the  roots  of  seedlings  are  more  sensitive 
to  inhibiting  factors  than  are  those  of  mature  plants. 

Oxygen. — There  was  little  evidence  of  unequal  distribution  of 
oxygen  through  the  dune  substratum.  The  only  exception  would 
be  in  connection  with  a high  water  table,  and  there  the  evidence 
was  not  conclusive.  The  slight  effects  observed  were  inhibitory  in 
nature,  and,  in  one  case  at  least,  referable  to  destructive  action  of 
micro-organisms  rather  than  to  a direct  reaction  of  the  plant  tissue 
to  the  absence  of  oxygen. 

P enetr ability  of  the  soil. — It  is  quite  possible  that  the  substratum 
may  be  more  penetrable  in  some  localities,  either  on  account  of 
differing  densities  of  certain  layers  or  of  the  disintegration  of  buried 
plant  parts.  This  would  be  difficult  to  prove  either  way,  but  there 
was  no  evidence  of  any  tendency  of  the  root  tips  to  be  turned  back 
by  a less  penetrable  layer  when  accidentally  wandering  too  near 
to  the  borders  of  a dark  layer,  as  would  be  the  case  if  the  dark  layer 
had  been  the  more  penetrable  medium.  In  fact  the  only  observed 
case  of  a probable  difference  in  penetrability  was  that  of  a soil 
layer  which  apparently  contained  a percentage  of  wind-blown  clay. 
In  this  case  certain  roots  were  distributed  along  the  upper  surface 
of  this  layer  when  normally  they  might  have  been  expected  to  pass 
directly  down  through  it.  As  already  indicated,  the  difference  of 
penetrability  of  the  moist  sand  under  the  soft  sand  mulch  may  be 
the  determining  factor  in  the  distribution  of  Artemisia  laterals. 
This  distribution,  however,  may  also  be  explained  by  the  difference 
in  moisture  content  of  the  two  layers  of  sand. 

There  is  no  clear  indication  as  to  the  method  by  which  the  chemi- 
cal substances  act  on  the  root,  whether  by  direct  stimulation  or  by 
removal  of  some  inhibition;  neither  are  the  relative  roles  of  organic 
and  inorganic  substances  more  than  suggested.  The  cases  of 
marked  elongation  of  roots  in  one  direction  would  seem  to  indicate 
the  possibility  that  roots  under  certain  conditions  can  make  use  of 


50 


BOTANICAL  GAZETTE 


[JULY 


organic  matter  directly;  in  other  words,  that  green  plants  may  be 
somewhat  saprophytic  in  nature.  A careful  microchemical  study 
of  the  contents  of  the  root  at  different  points  along  its  length  would 
be  necessary  in  order  to  get  any  definite  information  on  this  point. 
It  would  probably  occur  only  when  the  root  found  materials  exactly 
suited  to  its  needs,  and  would  be  proportionately  greater  as  the 
increasing  length  of  the  root  made  transportation  to  the  shoot 
increasingly  difficult. 

Under  these  stimulating  influences  abnormal  lengthening  and 
thickening  of  roots  occur  to  such  an  extent  as  to  call  in  question  the 
value  of  the  common  method  of  estimating  root  development  by 
measuring  the  length  and  weight  of  roots.  In  fact,  if  the  explana- 
tion of  direct  local  use  of  organic  material  of  the  root  be  accepted, 
the  abnormally  long  root  may  be  a detriment  instead  of  a benefit 
to  the  plant  as  a whole.  It  would  seem  as  though  there  could  be 
developed  some  method  of  evaluating  the  absorbing  power  of  roots 
through  study  of  the  structure  of  the  different  parts  of  the  root 
system  which  would  give  more  dependable  results  than  the  length 
and  weight  method. 

The  evidence  cited  emphasizes  the  unique  character  of  the 
dune  substratum,  in  that  pure  dune  sand  is  the  only  soil  in  which 
mineral  salts,  v/ith  the  exception  of  calcium  carbonate,  are  prac- 
tically absent,  and  organic  matter  is  so  rare  and  scattered  that  as  a 
general  factor  it  is  practically  negligible.  This  is  in  strong  con- 
trast with  many  arid  deserts  in  which  large  quantities  of  desirable 
mineral  salts  are  present,  needing  only  the  addition  of  water  to 
make  them  available  for  plant  use.  While  irrigation  is  the  main 
need  of  many  desert  stretches,  it  would  not  solve  the  problem  of 
plant  culture  on  a dune  area. 

Summary 

I.  The  study  of  the  literature  shows  that  the  work  done  on 
extension  and  development  of  root  systems  has  been  surprisingly 
little,  in  view  of  the  importance  of  the  root  in  the  utilization  of 
moisture  and  chemicals  in  the  soil.  This  study  also  emphasizes 
the  necessity  of  interpreting  the  extension  of  root  systems  in  the 
light  not  only  of  structure  and  functions,  but  also  of  the  causes  of 
such  extension. 


1919] 


WATERMAN— ROOT  SYSTEMS 


51 


2.  The  study  of  the  habitat  selected  shows  that  dune  sand  as  a 
substratum  for  plant  growth  is  almost  unique  in  uniformity  of 
texture  and  in  absence  of  mineral  salts  required  by  growing  plants. 
It  is  homogeneous  chemically,  but  contains  not  only  old  soil  layers 
but  minute  streaks  and  patches,  apparently  of  carbonaceous  and 
organic  origin,  as  well  as  dead  plant  parts,  very  unequally  dis- 
tributed. 

3.  The  roots  of  dune  species  react  differently  to  the  elements  of 
this  heterogeneous  structure,  extension  being  increased  in  some 
species  by  the  buried  organic  matter,  while  others  seem  unaffected 
or  even  inhibited  by  it. 

4.  These  reactions  are  specific  and  hereditary,  and  may  reflect 
the  conditions  under  which  the  ancestral  plants  grew'.  They  must 
be  regarded  as  of  great  importance  in  the  choosing  of  species  for 
introduction  into  conditions  where  the  humous  content  is  uneven. 

5.  Giving  due  weight  to  the  possibility  of  moisture,  oxygen 
content,  and  penetrability  of  the  sand  as  influencing  factors,  the 
evidence  seems  to  point  conclusively  to  nutrient  or  at  least  chemical 
influence  as  the  cause  of  variability  in  symmetry  in  the  extension 
of  roots  under  dune  conditions. 

6.  Under  certain 'conditions  the  root  apparently  utilizes  organic 
matter  directly,  at  the  expense  of  its  shoots.  Extreme  lengthening 
and  thickening  of  roots  occurring  under  these  conditions  call  into 
question  the  value  of  the  common  method  of  estimating  plant 
growth  by  measuring  the  length  and  weight  of  roots. 

Northwestern  University 
Evanston,  III. 

LITERATURE  CITED 

1.  Alien,  H.  von,  Wurzelstudien.  Bot.  Zeit.  67:175-199.  1909. 

2.  Benecke,  W.,  tiber  die  Keimung  der  Brutknospen  von  Lunaria  cruciata. 

Bot.  Zeit.  61:19-46.  1903. 

3.  Bennett,  M.  E.,  Are  roots  aerotropic  ? Bot.  Gaz.  37:241-259.  1904. 

4.  BtiSGEN,  M.,  Studien  iiber  die  Wurzelsysteme  einigen  dicotyler  Holz- 

pflanzen.  Flora  95:58-94.  1905. 

5.  , Ban  und  Leben  unserer  Waldbaiime.  Jena.  1897. 

6.  Cannon,  W.  A.,  Root  habits  of  desert  plants.  Carnegie  Publ.  131.  1911. 

7.  , Treelessness  in  prairie  regions.  Carnegie  Yearbook  12:71,  72. 


1913- 


52 


BOTANICAL  GAZETTE 


[JULY 


8.  Cannon,  W.  A.,  Physical  relation  of  roots  to  soil  factors.  Carnegie 
Yearbook  ii:6i,  62.  1912. 

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10.  , Evaluation  of  the  soil  temperature  factor  in  root  growth.  Plant 

World  21 164-69.  1918. 

11.  Cannon,  W.  A.,  and  Free,  E.  E.,  The  ecological  significance  of  soil  aera- 
tion. Science  N.S.  45: 178-180.  1917. 

12.  Craig,  J.,  Root  killing  of  forest  trees.  Iowa  Exper.  Sta.  Bull.  no.  44.  1899. 

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46:130-143.  1908. 

14.  , Bog  toxins.  Box.  Gaz.  47:389-405.  1909. 

15.  , Cranberry  Island.  Box.  Gaz.  52:1-33.  1911. 

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17.  , Studien  fiber  die  Wurzeln  krautiger  Pflanzen.  II.  Bibl.  Bot. 

61:  1904. 

18.  Goff,  E.  S.,  Study  of  roots  of  certain  perennial  plants.  Wis.  Agric.  Exper. 
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19.  Hellriegel,  H.,  Beitrage  zu  den  Grundlagen  des  Ackerbaus.  Braun- 
schweig. 1883. 

20.  Hilgard,  E.  W.,  Soils.  1906. 

21.  Hixchcock,  a.  S.,  Studies  on  subterranean  organs.  I and  II.  Trans. 
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22.  Hoveler,  W.,  tiber  die  Verwerthung  des  Humus  bei  der  Ernahrung  der 
chlorophyllfiihrenden  Pflanzen.  Jahrb.  Wiss.  Bot.  24:283-315.  1892. 

23.  King,  F.  H.,  Natural  distribution  of  roots  in  field  soils.  Wis.  Sta.  Report 
1892,  112;  and  1893,  160. 

24.  Kroemer,  K.,  Wurzelhaut,  Hypodermis,  und  Endodermis  der  Angio- 
spermenwurzeln.  Bibl.  Bot.  59: 1-151.  1903. 

25.  Nobbe,  F.,  tiber  die  feinere  Verastelung  der  Pflanzenwurzeln.  Landw. 
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26.  Noyes,  H.  A.,  Trosx,  J.  F.,  and  Yoder,  L.,  Root  variations  induced  by 
carbon  dioxide  gas  additions  to  soil.  Box.  Gaz.  66:364-373.  1918. 

27.  Pammel,  L.  H.,  Weeds  of  the  farm  and  garden.  New  York.  1911. 

28.  Pulling,  H.  E.,  Root  habit  and  distribution  in  the  far  north.  Plant  World 
21:223-233.  1918. 

29.  Rigg,  G.  B.,  Decay  and  soil  toxins.  Box.  Gaz.  61:295-310.  1916. 

30.  Rimbach,  a..  Contractile  roots.  Abstr.  in  Bot.  Centralbl.  74:209-211. 
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31.  Roxmisxrov,  V.,  Nature  of  drought.  Relation  of  root-  systems  to  soil’and 
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32.  Rusche,  E.,  Beeinflussung  der  Keimfahigkeit  verschiedener  Kulturpflanzen 
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iQig] 


IV  A TERM  A N—ROO  T S YS  TEMS 


53 


33.  Schreiner,  O.,  and  Reed,  H.  S.,  Excretions  by  roots.  Bull.  Torr.  Bot. 
Club  34:279-303.  1907. 

34.  Schulze,  B.,  Wurzelatlas.  Berlin.  1911. 

35.  Seelhorst,  C.  von,  Beobachtungen  liber  die  Zahl  und  den  Tiefgang  der 
Wurzeln  /verschiedener  Pflanzen  bei  verschiedener  Diingung  des  Bodens. 

• Jour.  Landw.  50:91-104.  1902. 

36.  Shepherd,  J.  B.,  Root  systems  of  field  crops.  N.D.  Station  Bull.  no.  64. 
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37.  Sherff,  E.  E.,  Vegetation  of  Skokie  Marsh.  Bull.  111.  Sta.  Lab.  575-614. 

1913- 

38.  Stohmann,  F.,  fiber  einige  Bedingungen  der  Vegetation  der  Pflanzen. 
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39.  Ten  Eyck,  W.  M.,  The  roots  of  plants.  Kansas  Exper.  Sta.  Agric.  Coll. 

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41.  Tottingham,  W.  E.,  Chemical  and  physiological  study  of  nutrient  solutions 
for  plant  culture.  Physiol.  Researches  1:133-245.  1914. 

42.  Transeau,  E.  N.,  Bogs  of  the  Huron  Valley.  Box.  Gaz.  40:351-418. 
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